A solid golf ball is a golf ball excepting a thread wound golf ball in which thread rubbers are wound on a core and covered. The solid golf ball includes a one piece solid golf ball obtained by one piece molding and a usual solid golf ball consisting of a solid core and a resin cover. The usual solid golf ball generally includes a two piece solid golf ball of which solid core is obtained by one piece molding, and a multipiece solid golf ball of which solid core comprising a center core and one or more layers coated thereon. The solid golf ball at least contains a resilient portion obtained by vulcanizing a rubber composition.
The rubber composition for the solid golf ball contains a base rubber (such as polybutadiene rubber) and a co-crosslinking agent (such as a monomer having an unsaturated bond, for instance, a metal salt of an unsaturated carboxylic acid and the like). The co-crosslinking agent is grafted or crosslinked to the polybutadiene chain by the function of a peroxide polymerization initiator to form a three dimensional structure of the polybutadiene and the monomer. The three dimensional structure imparts a necessary hardness, rebound properties and durability to the obtained solid golf ball.
Although the obtained golf ball has tolerable properties, it is further required for a golf ball to have higher rebound coefficient and durability. Improvement of rebound coefficient and durability has been made by means of an amount of the co-crosslinking agent, an amount of the peroxide and a vulcanizing temperature and the like. However, no golf balls perfectly meeting these the requirements have been obtained yet.
For further improvement of rebound properties and durability, the inventors have studied in view of raw materials, especially the base rubber, i.e. polybutadiene. It has been found that a high cis-polybutadiene rubber having a Mooney viscosity of at least 45, preferably 50 to 70 enhances the rebound properties and durability of the solid golf ball as long as it has the same hardness. This cis-polybutadiene rubber has a higher molecular weight than a conventional high cis-polybutadiene rubber which has a Mooney viscosity of 35 to 45. On the other hand, when the high cis-polybutadiene has a high Mooney viscosity, it adversely affects workability in mixing, molding etc, so as to result in poor quality with regard to stability of golf balls. It is surprising that high cis-polybutadiene rubber having a high Mooney viscosity does not deteriorate workability and simultaneously accomplishes an improvement in physical properties, if the polybutadiene rubber has a dispersity (dispersity=weight average molecular weight (Mw)/number average molecular weight (Mn)) of 4.0 to 8.0.